diff --git a/internal/plans/objchange/plan_valid.go b/internal/plans/objchange/plan_valid.go
index 5438d89d2e..6e8941fa02 100644
--- a/internal/plans/objchange/plan_valid.go
+++ b/internal/plans/objchange/plan_valid.go
@@ -358,10 +358,17 @@ func assertPlannedObjectValid(schema *configschema.Object, prior, config, planne
 		// both support a similar-enough API that we can treat them the
 		// same for our purposes here.
 
-		plannedL := planned.LengthInt()
-		configL := config.LengthInt()
-		if plannedL != configL {
-			errs = append(errs, path.NewErrorf("count in plan (%d) disagrees with count in config (%d)", plannedL, configL))
+		plannedL := planned.Length()
+		configL := config.Length()
+
+		// config wasn't known, then planned should be unknown too
+		if !plannedL.IsKnown() && !configL.IsKnown() {
+			return errs
+		}
+
+		lenEqual := plannedL.Equals(configL)
+		if !lenEqual.IsKnown() || lenEqual.False() {
+			errs = append(errs, path.NewErrorf("count in plan (%#v) disagrees with count in config (%#v)", plannedL, configL))
 			return errs
 		}
 		for it := planned.ElementIterator(); it.Next(); {
@@ -388,6 +395,20 @@ func assertPlannedObjectValid(schema *configschema.Object, prior, config, planne
 		configVals := map[string]cty.Value{}
 		priorVals := map[string]cty.Value{}
 
+		plannedL := planned.Length()
+		configL := config.Length()
+
+		// config wasn't known, then planned should be unknown too
+		if !plannedL.IsKnown() && !configL.IsKnown() {
+			return errs
+		}
+
+		lenEqual := plannedL.Equals(configL)
+		if !lenEqual.IsKnown() || lenEqual.False() {
+			errs = append(errs, path.NewErrorf("count in plan (%#v) disagrees with count in config (%#v)", plannedL, configL))
+			return errs
+		}
+
 		if !planned.IsNull() {
 			plannedVals = planned.AsValueMap()
 		}
@@ -421,10 +442,17 @@ func assertPlannedObjectValid(schema *configschema.Object, prior, config, planne
 		}
 
 	case configschema.NestingSet:
-		plannedL := planned.LengthInt()
-		configL := config.LengthInt()
-		if plannedL != configL {
-			errs = append(errs, path.NewErrorf("count in plan (%d) disagrees with count in config (%d)", plannedL, configL))
+		plannedL := planned.Length()
+		configL := config.Length()
+
+		// config wasn't known, then planned should be unknown too
+		if !plannedL.IsKnown() && !configL.IsKnown() {
+			return errs
+		}
+
+		lenEqual := plannedL.Equals(configL)
+		if !lenEqual.IsKnown() || lenEqual.False() {
+			errs = append(errs, path.NewErrorf("count in plan (%#v) disagrees with count in config (%#v)", plannedL, configL))
 			return errs
 		}
 		// Because set elements have no identifier with which to correlate
diff --git a/internal/plans/objchange/plan_valid_test.go b/internal/plans/objchange/plan_valid_test.go
index 8525e1431a..316bf56133 100644
--- a/internal/plans/objchange/plan_valid_test.go
+++ b/internal/plans/objchange/plan_valid_test.go
@@ -1692,6 +1692,115 @@ func TestAssertPlanValid(t *testing.T) {
 			}),
 			nil,
 		},
+
+		// When validating collections we start by comparing length, which
+		// requires guarding for any unknown values incorrectly returned by the
+		// provider.
+		"nested collection attrs planned unknown": {
+			&configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"set": {
+						Computed: true,
+						Optional: true,
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingSet,
+							Attributes: map[string]*configschema.Attribute{
+								"name": {
+									Type:     cty.String,
+									Computed: true,
+									Optional: true,
+								},
+							},
+						},
+					},
+					"list": {
+						Computed: true,
+						Optional: true,
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingList,
+							Attributes: map[string]*configschema.Attribute{
+								"name": {
+									Type:     cty.String,
+									Computed: true,
+									Optional: true,
+								},
+							},
+						},
+					},
+					"map": {
+						Computed: true,
+						Optional: true,
+						NestedType: &configschema.Object{
+							Nesting: configschema.NestingMap,
+							Attributes: map[string]*configschema.Attribute{
+								"name": {
+									Type:     cty.String,
+									Computed: true,
+									Optional: true,
+								},
+							},
+						},
+					},
+				},
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"set": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+				"list": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+				"map": cty.MapVal(map[string]cty.Value{
+					"key": cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"set": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+				"list": cty.SetVal([]cty.Value{
+					cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+				"map": cty.MapVal(map[string]cty.Value{
+					"key": cty.ObjectVal(map[string]cty.Value{
+						"name": cty.StringVal("from_config"),
+					}),
+				}),
+			}),
+			// provider cannot override the config
+			cty.ObjectVal(map[string]cty.Value{
+				"set": cty.UnknownVal(cty.Set(
+					cty.Object(map[string]cty.Type{
+						"name": cty.String,
+					}),
+				)),
+				"list": cty.UnknownVal(cty.Set(
+					cty.Object(map[string]cty.Type{
+						"name": cty.String,
+					}),
+				)),
+				"map": cty.UnknownVal(cty.Map(
+					cty.Object(map[string]cty.Type{
+						"name": cty.String,
+					}),
+				)),
+			}),
+			[]string{
+				`.set: count in plan (cty.UnknownVal(cty.Number)) disagrees with count in config (cty.NumberIntVal(1))`,
+				`.list: count in plan (cty.UnknownVal(cty.Number)) disagrees with count in config (cty.NumberIntVal(1))`,
+				`.map: count in plan (cty.UnknownVal(cty.Number)) disagrees with count in config (cty.NumberIntVal(1))`,
+			},
+		},
 	}
 
 	for name, test := range tests {